The present invention relates to a coded modulation scheme and, more particularly, to an improvement in coding rate conversion in a coded modulation scheme designed to compensate for redundancy by band expansion.
Recently, in digital microwave communication schemes, forward error correction schemes with higher error-correcting abilities have been introduced to improve transmission characteristics. One of such schemes is a coded modulation scheme. The coded modulation scheme incorporates an error-correcting technique and a modulation/demodulation technique. This scheme is superior to a conventional error-correcting scheme, which is independent of a modulation/demodulation technique. Note that in the conventional error-correcting scheme, redundancy bits are added to the respective signal streams in the same ratio, and rate conversion is performed for each stream.
Similar to the conventional error-correcting scheme, this coded modulation scheme adds redundancy signals to information signals (which are to be transmitted) so as to have an error-correcting ability. The scheme is different from the conventional scheme in that redundancy is compensated by an increase in the number of signal points instead of by band expansion. If, however, band expansion is allowed to a certain extent, the addition of redundancy by means of band expansion is advantageous in terms of information transmission efficiency. In this case, rate converters for performing rate conversion for each frame must be arranged in both transmitting and receiving sections.
In the coded modulation scheme, since processing for coding varies for each signal stream, rate conversion must be performed collectively in minimum units of signals input to an encoder. In an SDH (Synchronous Digital Hierarchy) transmission system, since signal processing must be performed for each byte, the number of signal streams input to an encoder is preferably eight. If, however, the number of input signal streams is limited to eight, it may be difficult to realize a rate converter depending on a coding rate R. If the coding rate R is given by R=N/M (N and M are natural numbers, and M is larger than N), and N is a multiple of eight, each input signal stream may be serial/parallel-converted at a one-to-one ratio (N/8), and a redundancy signal may be added to the resultant signal. If, however, N is not a multiple of eight but is a multiple of four, rate conversion in coding must be performed collectively with respect to signals of two minimum units. That is, one block is constituted by two minimum units. In rate conversion on the reception side, parallel/serial conversion is required. If the delimiters of blocks are not defined, the problem of uncertainty is posed in the arrangement of signals in parallel/serial conversion. No information about the delimiters of blocks is obtained from a decoder. In general, therefore, in order to detect the delimiters of blocks, frame information other than an information signal and a redundancy signal obtained by error-correcting coding, i.e., a frame pulse for frame synchronization, is inserted between blocks in a transmitting section, and the frame pulse is detected in a receiving section ("Digital Transmission System", pp. 131-141, 1985).
In the conventional coded modulation scheme described above, however, in order to eliminate the uncertainty of a rate conversion unit in the receiving section, phase information, i.e., a frame pulse, must be added to information signal bits. For this reason, in the transmitting section, the information signal transmission efficiency deteriorates in accordance with the insertion of the frame pulse. Furthermore, the receiving section requires a frame synchronization circuit as an extra component.